Polarized distribution of key membrane transport proteins in the rat submandibular gland

Rescue of non-healing, degenerative salivary glands by cholinergic-calcium signaling

Chronic degenerative wounds are often deemed irreparable, directing research efforts to focus predominantly on acute tissue injury regeneration while leaving endogenous repair mechanisms for chronically damaged tissues largely unexplored. In this study, we demonstrate that non-healing, severely degenerated salivary gland tissues can be fundamentally restored through first-line treatment with muscarinic agonists. This approach rescues tissue structure and function, returning it to a homeostatic-like state, and reactivates endogenous regeneration processes to drive new cell expansion that persists for months post-treatment. Furthermore, neuromimetic activation profoundly depletes radiation-induced DNA damage and re-establishes the nerve-acinar relationship, ultimately restoring the tissues physiological capacity to maintain homeostasis, even in the absence of treatment. We show that full recovery of organ function, comparable to uninjured controls, is primarily mediated by the re-differentiation of aberrantly de-differentiated epithelial acinar cells and the restoration of mitochondrial function via a muscarinic-calcium signaling pathway. These findings challenge the prevailing notion that chronic organ degeneration is irreversible and propose a readily testable therapeutic strategy for epithelial restoration with potential applications across a spectrum of chronic injuries.

Association Between Genetically Proxied SLC12A2 Inhibition and Inflammatory Bowel Disease: A Mendelian Randomization Study

The global rise in hypertension prompts the use of medications to manage blood pressure. However, selecting first-line drugs remains challenging as their efficacy often stems from blood pressure reduction rather than specific pharmacological actions. Evaluating interactions between antihypertensive drugs and common diseases can aid tailored treatment. Here, we assess the potential link between antihypertensives and inflammatory bowel disease (IBD). Summary-level coronary heart disease (CHD) data (184,305 individuals), systolic BP (SBP) data (757,601 individuals), ulcerative ileocolitis data (361,188 individuals), ulcerative colitis data (364,454 individuals), other ulcerative colitis data (361,619 individuals), and ulcerative proctitis data (361,700 individuals) were all from genome-wide association studies (GWASs), FinnGen or eQTL studies publicly accessible. The DrugBank10 and ChEMBL11 databases function to identify genes encoding protein products targeted by active constituents of BP-lowering drugs. Summary-data-based MR (SMR) estimated the associations between expressions of drug target genes and symptoms of IBD. A multivariable MR study was further conducted to examine if the observed association was direct association. Subsequently, we collected blood samples from IBD patients in the Gastroenterology Department of Gastroenterology, the First Affiliated Hospital of Anhui Medical University and blood from healthy individuals at the physical examination center. Real-time quantitative PCR was employed to detect the expression changes of drug target genes in the peripheral blood of patients with IBD. Furthermore, we used Caco2 cells to construct an in vitro model of IBD, examined the expression of the target molecules, and verified the potential of Bumetanide to improve IBD. SMR analysis revealed that enhanced SLC12A2 gene expression in blood (equivalent to a one standard deviation increase) was a risk factor for ulcerative ileocolitis (beta = 0.5861, se = 0.2972, p = 0.0486) and enhanced gene expression of ACE was a protective factor. Additionally, SCNN1D and SLC16A1 played protective roles of IBD, while NR3C1 was identified as a risk factor. However, among these genes, only SLC12A2 was considered to influence the progress of inflammatory bowel disease through systolic blood pressure based on Mendelian randomization analysis results. Other genes may be associated with IBD depending on the expression of their own proteins, independent of changes in blood pressure. In the peripheral blood of IBD patients and in vitro experiments, SCL12A2 has been shown to be highly expressed in IBD. In vitro experiments have confirmed that Bumetanide can inhibit SCL12A2 to improve tight junctions, reduce inflammation levels, and ameliorate IBD symptoms. Therapeutic inhibition of SCL12A2 may benefit patients with IBD. In the future, this study may contribute to the selection of more personalized antihypertensive medications for different subgroups of hypertensive patients.

Structural and functional analysis of salivary intercalated duct cells reveals a secretory phenotype

Currently, all salivary ducts (intercalated, striated and collecting) are assumed to function broadly in a similar manner, reclaiming ions that were secreted by the secretory acinar cells while preserving fluid volume and delivering saliva to the oral cavity. Nevertheless, there has been minimal investigation into the structural and functional differences between distinct types of salivary duct cells. Therefore, in this study, the expression profile of proteins involved in stimulus-secretion coupling, as well as the function of the intercalated duct (ID) and striated duct cells, was examined. Particular focus was placed on defining differences between distinct duct cell populations. To accomplish this, immunohistochemistry and in situ hybridization were utilized to examine the localization and expression of proteins involved in reabsorption and secretion of ions and fluid. Further, in vivo calcium imaging was employed to investigate cellular function. Based on the protein expression profile and functional data, marked differences between the IDs and striated ducts were observed. Specifically, the ID cells express proteins native to the secretory acinar cells while lacking proteins specifically expressed in the striated ducts. Further, the ID and striated duct cells display different calcium signalling characteristics, with the IDs responding to a neural stimulus in a manner similar to the acinar cells. Overall, our data suggest that the IDs have a distinct role in the secretory process, separate from the reabsorptive striated ducts. Instead, based on our evidence, the IDs express proteins found in secretory cells, generate calcium signals in a manner similar to acinar cells, and, therefore, are likely secretory cells. KEY POINTS: Current studies examining salivary intercalated duct cells are limited, with minimal documentation of the ion transport machinery and the overall role of the cells in fluid generation. Salivary intercalated duct cells are presumed to function in the same manner as other duct cells, reclaiming ions, maintaining fluid volume and delivering the final saliva to the oral cavity. Here we systematically examine the structure and function of the salivary intercalated duct cells using immunohistochemistry, in situ hybridization and by monitoring in vivo Ca2+ dynamics. Structural data revealed that the intercalated duct cells lack proteins vital for reabsorption and express proteins necessary for secretion. Ca2+ dynamics in the intercalated duct cells were consistent with those observed in secretory cells and resulted from GPCR-mediated IP3 production.

Insights into the Function of Aquaporins in Gastrointestinal Fluid Absorption and Secretion in Health and Disease

Aquaporins (AQPs), transmembrane proteins permeable to water, are involved in gastrointestinal secretion. The secretory products of the glands are delivered either to some organ cavities for exocrine glands or to the bloodstream for endocrine glands. The main secretory glands being part of the gastrointestinal system are salivary glands, gastric glands, duodenal Brunner's gland, liver, bile ducts, gallbladder, intestinal goblet cells, exocrine and endocrine pancreas. Due to their expression in gastrointestinal exocrine and endocrine glands, AQPs fulfill important roles in the secretion of various fluids involved in food handling. This review summarizes the contribution of AQPs in physiological and pathophysiological stages related to gastrointestinal secretion.

Aquaporin-1 inhibition exacerbates ischemia-reperfusion-induced lung injury in mouse

BACKGROUND

Ischemia-reperfusion injury (IRI), which involves severe inflammation and edema, is an inevitable feature of the lung transplantation process and leads to primary graft dysfunction (PGD). The activation of aquaporin 1 (AQP1) modulates fluid transport in the alveolar space. The current study investigated the role of AQP1 in ischemia-reperfusion (IR)-induced lung injury.

METHODS

A mouse model of lung IR was established by clamping the left lung hilar for 1 h and released for reperfusion for 24 h. The AQP1 inhibitor acetazolamide (AZA) was administered 3 days before lung ischemia with a dose of 100 mg/kg per day via gavage. Lung injury was evaluated using the ratio of wet-to-dry weight, peripheral bronchial epithelial thickness, degree of angioedema, acute lung injury score, neutrophil infiltration, and cytokine concentrations in bronchoalveolar lavage fluid.

RESULTS

Compared with sham treatment, ischemia with no reperfusion (IR 0h) and ischemia with reperfusion for 24 h (IR 24 h) significantly upregulated AQP1 expression, increased the wet/dry weight ratio, angioedema, neutrophil infiltration and cytokine production (interleukin -6 and tumor necrosis factor -α) and thickened the peripheral bronchial epithelium. AZA exacerbated inflammation and pulmonary edema.

CONCLUSION

AQP1 may exert a protective effect against IR-induced lung injury, which could be attributed to alleviating pulmonary edema and inflammation. AQP1 upregulation might be a potential application to alleviate lung IRI and decrease the incidence of PGD.

Aquaporins in Glandular Secretion

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.

Ablation of AQP5 gene in mice leads to olfactory dysfunction caused by hyposecretion of Bowman's gland

Smell detection depends on nasal airflow, which can make absorption of odors to the olfactory epithelium by diffusion through the mucus layer. The odors then act on the chemo-sensitive epithelium of olfactory sensory neurons (OSNs). Therefore, any pathological changes in the olfactory area, for instance, dry nose caused by Sjögren's Syndrome (SS) may interfere with olfactory function. SS is an autoimmune disease in which aquaporin (AQP) 5 autoantibodies have been detected in the serum. However, the expression of AQP5 in olfactory mucosa and its function in olfaction is still unknown. Based on the study of the expression characteristics of AQP5 protein in the nasal mucosa, the olfaction dysfunction in AQP5 knockout (KO) mice was found by olfactory behavior analysis, which was accompanied by reduced secretion volume of Bowman's gland by using in vitro secretion measure system, and the change of acid mucin in nasal mucus layer was identified. By excluding the possibility that olfactory disturbance was caused by changes in OSNs, the result indicated that AQP5 contributes to olfactory functions by regulating the volume and composition of OE mucus layer, which is the medium for the dissolution of odor molecules. Our results indicate that AQP5 can affect the olfactory functions by regulating the water supply of BGs and the mucus layer upper the OE that can explain the olfactory loss in the patients of SS, and AQP5 KO mice might be used as an ideal model to study the olfactory dysfunction.

Physiological Cooperation between Aquaporin 5 and TRPV4

Aquaporins—among them, AQP5—are responsible for transporting water across biological membranes, which is an important process in all living organisms. The transient receptor potential channel 4 (TRPV4) is a cation channel that is mostly calcium-permeable and can also be activated by osmotic stimuli. It plays a role in a number of different functions in the body, e.g., the development of bones and cartilage, and it is involved in the body’s osmoregulation, the generation of certain types of sensation (pain), and apoptosis. Our earlier studies on the uterus and the literature data aroused our interest in the physiological role of the cooperation of AQP5 and TRPV4. In this review, we focus on the co-expression and cooperation of AQP5 and TRPV4 in the lung, salivary glands, uterus, adipose tissues, and lens. Understanding the cooperation between AQP5 and TRPV4 may contribute to the development of new drug candidates and the therapy of several disorders (e.g., preterm birth, cataract, ischemia/reperfusion-induced edema, exercise- or cold-induced asthma).

A Mathematical Model of Salivary Gland Duct Cells

Saliva is produced in two stages in the salivary glands: the secretion of primary saliva by the acinus and the modification of saliva composition to final saliva by the intercalated and striated ducts. In order to understand the saliva modification process, we develop a mathematical model for the salivary gland duct. The model utilises the realistic 3D structure of the duct reconstructed from an image stack of gland tissue. Immunostaining results show that TMEM16A and aquaporin are expressed in the intercalated duct cells and that ENaC is not. Based on this, the model predicts that the intercalated duct does not absorb Na\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^+$$\end{document}+ and Cl\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^-$$\end{document}- like the striated duct but secretes a small amount of water instead. The input to the duct model is the time-dependent primary saliva generated by an acinar cell model. Our duct model produces final saliva output that agrees with the experimental measurements at various stimulation levels. It also shows realistic biological features such as duct cell volume, cellular concentrations and membrane potentials. Simplification of the model by omission of all detailed 3D structures of the duct makes a negligible difference to the final saliva output. This shows that saliva production is not sensitive to structural variation of the duct.

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

A Cardioplegic Solution with an Understanding of a Cardiochannelopathy

Cardiac surgeries have been improved by accompanying developing cardioplegia solutions. However, the cardioplegia application presents an ongoing challenge with a view of a sufficiently restored cardiac function. In this review, we focus on the cardioplegia-induced mechanism and summarize the findings of studies undertaken to improve cardioprotective strategies. Currently, and somewhat surprisingly, relatively little is known about cardiac electrolyte regulation through channel physiology. We hope that an improved understanding of the electrolyte transport through ion channels/transporters and modulations of water channel aquaporins will provide an insight into cardiac channel physiology and a channel-based cardiac pathology of a cardiochannelopathy.

NKCC1, an Elusive Molecular Target in Brain Development: Making Sense of the Existing Data

Ionotropic GABA transmission is mediated by anion (mainly Cl⁻)-permeable GABA_A receptors (GABA_ARs). In immature neurons, GABA exerts depolarizing and sometimes functionally excitatory actions, based on active uptake of Cl⁻ by the Na-K-2Cl cotransporter NKCC1. While functional evidence firmly shows NKCC1-mediated ion transport in immature and diseased neurons, molecular detection of NKCC1 in the brain has turned out to be extremely difficult. In this review, we describe the highly inconsistent data that are available on the cell type-specific expression patterns of the NKCC1 mRNA and protein in the CNS. We discuss the major technical caveats, including a lack of knock-out-controlled immunohistochemistry in the forebrain, possible effects of alternative splicing on the binding of antibodies and RNA probes, and the wide expression of NKCC1 in different cell types, which make whole-tissue analyses of NKCC1 useless for studying its neuronal expression. We also review novel single-cell RNAseq data showing that most of the NKCC1 in the adult CNS may, in fact, be expressed in non-neuronal cells, especially in glia. As future directions, we suggest single-cell NKCC1 mRNA and protein analyses and the use of genetically tagged endogenous proteins or systematically designed novel antibodies, together with proper knock-out controls, for the visualization of endogenous NKCC1 in distinct brain cell types and their subcellular compartments.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Novel Human NKCC1 Mutations Cause Defects in Goblet Cell Mucus Secretion and Chronic Inflammation

BACKGROUND & AIMS

Infections resulting from intestinal yeast and bacteria affect a large number of patients with deficits in absorptive or secretory epithelial transport mechanisms. The basolateral Na⁺-K⁺-2Cl^- cotransporter (NKCC1) has been implicated in intestinal epithelial fluid secretion. Two patients with deleterious heterozygous (NKCC1-DFX, DFX for Asp-Phe-stop codon) or homozygous (Kilquist) mutations in SLC12A2 (NKCC1) suffered from gastrointestinal deficits. Because of chronic infections, the colon and the small intestine of the NKCC1-DFX patient were resected surgically.

METHODS

To investigate how NKCC1 affects the integrity and function of the gut epithelia, we used a mouse model recapitulating the NKCC1-DFX patient mutation. Electron microscopy and immunostaining were used to analyze the integrity of the colonic mucus layers and immune cell infiltration. Fluorescence in situ hybridization was performed on the distal colon sections to measure bacteria translocation to the mucosa and submucosa. Citrobacter rodentium was used to measure mouse ability to clear enteric infection. A multiplex cytokine assay was used to analyze mouse inflammatory response to infection.

RESULTS

We show that NKCC1-DFX expression causes defective goblet cell mucus granule exocytosis, leading to secretion of intact granules into the lumen of the large intestine. In addition, NKCC1-DFX colon submucosal glands secrete mucus that remained attached to the epithelium. Importantly, expression of the mutant NKCC1 or complete loss of NKCC1 function leads to aggravated inflammatory response to C rodentium infection. Compared with wild-type, NKCC1-DFX mice showed decreased expression of claudin-2, a tight junction protein involved in paracellular Na⁺ and water transport and enteric infection clearance.

CONCLUSIONS

Our data indicate that NKCC1-DFX impairs gut barrier function by affecting mucus secretion and immune properties.

Long-term Pilocarpine Treatment Improves Salivary Flow in Irradiated Mice

Radiation therapy for head and neck cancer frequently causes salivary gland dysfunction. Pilocarpine is a clinically approved and effective drug that induces saliva secretion, thereby keeping the oral mucosa moist and reducing discomfort in patients, but the effect is transient. We expected that this drug also has beneficial long-term effects that maintain the integrity of salivary glands by reducing, for instance, apoptosis. Here, we examined the effects of long-term pilocarpine administration in irradiated mice. The results indicated that long-term pilocarpine administration significantly improved salivary flow in irradiated mice, suggesting the potential beneficial effects of long-term administration. To elucidate the underlying mechanism, we analyzed the histology, apoptosis, and proliferation of acinar cells, and the expression of functional membrane proteins such as transmembrane member 16A, aquaporin-5, and Na-K-Cl cotransporter. Long-term pilocarpine treatment seemed to decrease irradiation-induced apoptosis, although the change was not statistically significant. The present results indicated that long-term administration of pilocarpine has beneficial effects on salivary flow in irradiated mice, and suggested that long-term administration possibly decreases apoptosis in irradiated salivary glands.

Different immunohistochemical localization for TMEM16A and CFTR in acinar and ductal cells of rat major salivary glands and exocrine pancreas

We investigated the mRNA expression and immunohistochemical localization of Cl^- channels, transmembrane member 16A (TMEM16A or anoctamin 1), and cystic fibrosis transmembrane conductance regulator (CFTR) in rat major salivary glands and exocrine pancreas. RT-PCR detected mRNA expression of TMEM16A and CFTR in the extracts of the parotid gland (PG), submandibular gland (SMG), sublingual gland (SLG), and pancreas. Immunoreactivity for TMEM16A was localized in the apical membrane of serous acinar and intercalated ductal cells in the PG and SMG as well as mucous acinar cells in the SLG; however, it was not detected in striated ductal cells of these tissues. Although striated ductal cells in the PG, SMG and SLG, and granular ductal cells in the SMG, were immunoreactive for CFTR in the luminal side, serous, mucous acinar, and intercalated ductal cells were not immunoreactive for CFTR in any of the major salivary glands. In the exocrine pancreas, immunoreactivity for TMEM16A was localized in the apical membrane of acinar cells, while immunoreactivity for CFTR was localized in the luminal side of intercalated ductal cells. These results suggest that different localization of TMEM16A and CFTR immunoreactivities reflects the respective functions of acinar and ductal cells in major salivary glands and exocrine pancreas.

Salivary secretion in health and disease

Saliva is a complex fluid produced by 3 pairs of major salivary glands and by hundreds of minor salivary glands. It comprises a large variety of constituents and physicochemical properties, which are important for the maintenance of oral health. Saliva not only protects the teeth and the oropharyngeal mucosa, it also facilitates articulation of speech, and is imperative for mastication and swallowing. Furthermore, saliva plays an important role in maintaining a balanced microbiota. Thus, the multiple functions provided by saliva are essential for proper protection and functioning of the body as a whole and for the general health. A large number of diseases and medications can affect salivary secretion through different mechanisms, leading to salivary gland dysfunction and associated oral problems, including xerostomia, dental caries and fungal infections. The first part of this review article provides an updated insight into our understanding of salivary gland structure, the neural regulation of salivary gland secretion, the mechanisms underlying the formation of saliva, the various functions of saliva and factors that influence salivary secretion under normal physiological conditions. The second part focuses on how various diseases and medical treatment including commonly prescribed medications and cancer therapies can affect salivary gland structure and function. We also provide a brief insight into how to diagnose salivary gland dysfunction.

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Daily voluntary exercise enhances pilocarpine-induced saliva secretion and aquaporin 1 expression in rat submandibular glands

Saliva—a water‐based fluid containing electrolytes, immunoglobulins, and enzymes—has many functions, including the protection and hydration of mucosal structures within the oral cavity and the initiation of digestion. Aquaporins (AQPs) are proteins that act as water channels through membranes. We have previously reported upregulation of the expression levels of AQP 1 and 5 in the submandibular glands (SMGs) in heat‐acclimated rats. In this study, we investigated pilocarpine‐induced saliva secretion and AQP expression in rats after voluntary exercise. Male, 10‐week‐old Wistar rats were initially maintained at an ambient temperature of 24 °C for 10 days and were then kept for 40 days in cages either with a running wheel (exercise rats, n = 6) or with a locked wheel [control rats (CN), n = 6]. After the training period, the rats were anesthetized and pilocarpine, an M3 muscarinic receptor agonist, was intraperitoneally injected (0.5 mg·kg⁻¹) to stimulate saliva secretion. Saliva was collected, and the SMGs were sampled and subjected to western blot, RT‐PCR, and immunohistochemical analyses. Pilocarpine induced a greater amount of saliva in the exercised rats than in the CN. Expression levels of AQP1 mRNA and protein were significantly higher in SMGs of exercised rats than in those of the CN, but the expression of AQP5 was not affected by voluntary exercise. Voluntary exercise increased the expression of vascular endothelial growth factor (VEGF) and cluster of differentiation 31 (CD31), a marker for endothelial cells, in the SMGs. Voluntary exercise promoted pilocarpine‐induced saliva secretion, probably via an increase in the expression level of AQP1 due to VEGF‐induced CD31‐positive angiogenesis in the SMG.

Aquaporins and Gland Secretion

Aquaporins (AQPs ) are expressed in most exocrine and endocrine secretory glands. Consequently, summarizing the expression and functions of AQPs in secretory glands represents a daunting task considering the important number of glands present in the body, as well as the number of mammalian AQPs - thirteen. The roles played by AQPs in secretory processes have been investigated in many secretory glands. However, despite considerable research, additional studies are clearly needed to pursue our understanding of the role played by AQPs in secretory processes. This book chapter will focus on summarizing the current knowledge on AQPs expression and function in the gastrointestinal tract , including salivary glands, gastric glands, Duodenal Brunner's gland, liver and gallbladder, intestinal goblets cells, exocrine and endocrine pancreas, as well as few other secretory glands including airway submucosal glands, lacrimal glands, mammary glands and eccrine sweat glands.

Co-localization of endogenous Arf6 and its activator EFA6D in the granular convoluted tubule cells of mouse submandibular glands under normal conditions and when stimulated by isoproterenol, noradrenaline and carbachol

OBJECTIVE

This study proposed to investigate the localization at light and electron microscopic levels of Arf6 and its activator EFA6D in the mouse submandibular gland (SMG) under normal conditions and when stimulated by adrenergic or cholinergic agonists.

MATERIALS AND METHODS

SMGs of male adult mice were utilized for immunoblotting and immuno-light and -electron microscopic analyses. Isoproterenol and noradrenalin were used as adrenergics, while carbachol was used for the cholinergic stimulant. SMGs were examined at 15, 30, 60 and 120min after intraperitoneal injection of these agents.

RESULTS

Immunoreactivities for both Arf6 and its activator EFA6D were similarly intense in the basolateral domain of GCTs, but no significant immunoreactivities were seen in the apical domain of GCT cells or any domain of acinar cells under normal conditions. In immuno-electron microscopy, the immunoreactive materials were mainly deposited on the basolateral plasma membranes and subjacent cytoplasm. Shortly after injection of isoproterenol and noradrenaline, but not carbachol, the immunoreactivities for both molecules were additionally seen on the apical plasmalemma of most, if not all, GCT cells, but not acinar cells.

CONCLUSION

The present findings suggest that the direct involvement of Arf6/EFA6D in regulatory exocytosis at the apical plasma membrane of acinar and GCT cells is apparently to be smaller, if present, than that of endocytosis at the basolateral membranes of GCT cells under normal conditions. This also suggests that the two molecules function additionally at the apical membrane of GCT cells for modulation of saliva secretion under β-adrenoceptor stimulation.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

Promising Gene Therapeutics for Salivary Gland Radiotoxicity

More than 0.5 million new cases of head and neck cancer are diagnosed worldwide each year, and approximately 75% of them are treated with radiation alone or in combination with other cancer treatments. A majority of patients treated with radiotherapy develop significant oral off-target effects because of the unavoidable irradiation of normal tissues. Salivary glands that lie within treatment fields are often irreparably damaged and a decline in function manifests as dry mouth or xerostomia. Limited ability of the salivary glands to regenerate lost acinar cells makes radiation-induced loss of function a chronic problem that affects the quality of life of the patients well beyond the completion of radiotherapy. The restoration of saliva production after irradiation has been a daunting challenge, and this review provides an overview of promising gene therapeutics that either improve the gland’s ability to survive radiation insult, or alternately, restore fluid flow after radiation. The salient features and shortcomings of each approach are discussed.

The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells-a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.

Aquaporins in Salivary Glands: From Basic Research to Clinical Applications

Salivary glands are involved in saliva secretion that ensures proper oral health. Aquaporins are expressed in salivary glands and play a major role in saliva secretion. This review will provide an overview of the salivary gland morphology and physiology of saliva secretion, and focus on the expression, subcellular localization and role of aquaporins under physiological and pathophysiological conditions, as well as clinical applications involving aquaporins. This review is highlighting expression and localization of aquaporins in human, rat and mouse, the most studied species and is pointing out possible difference between major salivary glands, i.e., parotid, submandibular and sublingual glands.

Opposing Effects of cAMP and T259 Phosphorylation on Plasma Membrane Diffusion of the Water Channel Aquaporin-5 in Madin-Darby Canine Kidney Cells

Aquaporin-5 (AQP5) facilitates passive water transport in glandular epithelia in response to secretory stimuli via intracellular pathways involving calcium release, cAMP and protein kinase A (PKA). In epithelial plasma membranes, AQP5 may be acutely regulated to facilitate water transport in response to physiological stimuli by changes in protein modifications, interactions with proteins and lipids, nanoscale membrane domain organization, and turnover rates. Such regulatory mechanisms could potentially be associated with alteration of diffusion behavior, possibly resulting in a change in the plasma membrane diffusion coefficient of AQP5. We aimed to test the short-term regulatory effects of the above pathways, by measuring lateral diffusion of AQP5 and an AQP5 phospho-mutant, T259A, using k-space Image Correlation Spectroscopy of quantum dot- and EGFP-labeled AQP5. Elevated cAMP and PKA inhibition significantly decreased lateral diffusion of AQP5, whereas T259A mutation showed opposing effects; slowing diffusion without stimulation and increasing diffusion to basal levels after cAMP elevation. Thus, lateral diffusion of AQP5 is significantly regulated by cAMP, PKA, and T259 phosphorylation, which could be important for regulating water flow in glandular secretions.

Potential Reparative Role of Resident Adult Renal Stem/Progenitor Cells in Acute Kidney Injury

Human kidney is particularly susceptible to ischemia and toxins with consequential tubular necrosis and activation of inflammatory processes. This process can lead to the acute renal injury, and even if the kidney has a great capacity for regeneration after tubular damage, in several circumstances, the normal renal repair program may not be sufficient to achieve a successful regeneration. Resident adult renal stem/progenitor cells could participate in this repair process and have the potentiality to enhance the renal regenerative mechanism. This could be achieved both directly, by means of their capacity to differentiate and integrate into the renal tissues, and by means of paracrine factors able to induce or improve the renal repair or regeneration. Recent genetic fate-tracing studies indicated that tubular damage is instead repaired by proliferative duplication of epithelial cells, acquiring a transient progenitor phenotype and by fate-restricted clonal cell progeny emerging from different nephron segments. In this review, we discuss about the properties and the reparative characteristics of high regenerative CD133(+)/CD24(+) cells, with a view to a future application of these cells for the treatment of acute renal injury.

Expression patterns of CD66a and CD117 in the mouse submandibular gland

The epithelial tissue of the salivary gland consists of the acinar and ductal parts, the latter of which is further divided into the intercalated, striated and excretory duct segments and is the residential site for salivary stem/progenitor cells. In the present study, the expression patterns of two cell surface molecules, CD66a and CD117, were investigated in the adult mouse submandibular glands (SMG) by immunofluorescence microscopy. Combinations of the two molecules differentially marked several types of SMG epithelial cells, including acinar cells (CD66a-intense, CD117-negative), intercalated duct cells (CD66a-intense, CD117-positive), a subset of the striated and excretory duct cells (CD66a-weak, CD117-positive). Most of the CD117-positive ductal cells were negative for cytokeratin 5 and overlapped with the NKCC1-expressing cells. The CD117- and keratin 5-positive cells resided only in the excretory duct were suggested to correspond to the recently identified salivary stem cells. CD66a and CD117 may be useful markers to isolate several cell types consisting of SMG epithelium and to analyze their molecular and cellular nature. Our data also suggest that CD117-expressing epithelial cells of the gland include at least two distinct populations of the stem/progenitor cells.

Human aquaporins: regulators of transcellular water flow

BACKGROUND

Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes.

SCOPE OF REVIEW

AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.

MAJOR CONCLUSIONS

AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.

GENERAL SIGNIFICANCE

Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Expression of the homeostasis-related markers, maspin, heat shock proteins 70 & 90, glutathione S-transferase, aquaporin 5 and NF-kB in young and old labial and palatal salivary glands

Intraoral salivary glands undergo remarkable age-related morphologic changes. This study investigated the expression of a panel of molecular markers known for cellular homeostatic activity, dependent on age and location of the salivary glands. Samples taken from healthy subjects were classified according to age ("young" <45 years, n=51, and "old" ≥60 years, n=45) and location (lip, n=47 and palate, n=49). They were immunohistochemically stained for mammary serine protease inhibitor (maspin), heat shock protein (HSP)70, HSP90, glutathione S-transferase (GST), aquaporine5 (AQP5), and nuclear factor kappa-B (NF-κB) for assessment of their expression in acini and ducts, and in cytoplasmic and nuclear compartments. Results were expressed as the mean percentage of positively stained component per age group, gland location and type of cell and cellular compartment. Statistical analysis was performed by two-way ANOVA and crosstabs. The expression of maspin was lower in the old group in both the palatal and labial glands (acini and ducts, cytoplasm and nuclei) compared to the young group (p<0.05). In both age groups, when compared to labial glands, palatal glands exhibited higher expression of HSP70 (p<0.05) and lower expression of AQP5 (p<0.001) and NF-κB (p=0.018). Collectively, the low expression of factors capable of preserving cellular homeostasis (i.e., maspin and AQP5) vis-à-vis a high expression of factors that are also related to cell survival (i.e., HSPs) that was demonstrated in the old palatal glands may point to their high vulnerability to undergo selective phenotypic changes.

Identification of moesin as NKCC2-interacting protein and analysis of its functional role in the NKCC2 apical trafficking

BACKGROUND INFORMATION

The renal Na(+) -K(+) -2Cl(-) co-transporter (NKCC2) is expressed in kidney thick ascending limb cells, where it mediates NaCl re-absorption regulating body salt levels and blood pressure.

RESULTS

In this study, we used a well-characterised NKCC2 construct (c-NKCC2) to identify NKCC2-interacting proteins by an antibody shift assay coupled with blue native/SDS-PAGE and mass spectrometry. Among the interacting proteins, we identified moesin, a protein belonging to ezrin, eadixin and moesin family. Co-immunoprecipitation experiments confirmed that c-NKCC2 interacts with the N-terminal domain of moesin in LLC-PK1 cells. Moreover, c-NKCC2 accumulates in intracellular and sub-apical vesicles in cells transfected with a moesin dominant negative green fluorescent protien (GFP)-tagged construct. In addition, moesin knock-down by short interfering RNA decreases by about 50% c-NKCC2 surface expression. Specifically, endocytosis and exocytosis assays showed that moesin knock-down does not affect c-NKCC2 internalisation but strongly reduces exocytosis of the co-transporter.

CONCLUSIONS

Our data clearly demonstrate that moesin plays a critical role in apical membrane insertion of NKCC2, suggesting a possible involvement of moesin in regulation of Na(+) and Cl(-) absorption in the kidney.

Cell culture models and animal models for studying the patho-physiological role of renal aquaporins

Aquaporins (AQPs) are key players regulating urinary-concentrating ability. To date, eight aquaporins have been characterized and localized along the nephron, namely, AQP1 located in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2, AQP3 and AQP4 in collecting duct principal cells; AQP5 in intercalated cell type B; AQP6 in intercalated cells type A in the papilla; AQP7, AQP8 and AQP11 in the proximal tubule. AQP2, whose expression and cellular distribution is dependent on vasopressin stimulation, is involved in hereditary and acquired diseases affecting urine-concentrating mechanisms. Due to the lack of selective aquaporin inhibitors, the patho-physiological role of renal aquaporins has not yet been completely clarified, and despite extensive studies, several questions remain unanswered. Until the recent and large-scale development of genetic manipulation technology, which has led to the generation of transgenic mice models, our knowledge on renal aquaporin regulation was mainly based on in vitro studies with suitable renal cell models. Transgenic and knockout technology approaches are providing pivotal information on the role of aquaporins in health and disease. The main goal of this review is to update and summarize what we can learn from cell and animal models that will shed more light on our understanding of aquaporin-dependent renal water regulation.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

AQP5 is expressed in type-B intercalated cells in the collecting duct system of the rat, mouse and human kidney

We screened human kidney-derived multipotent CD133+/CD24+ ARPCs for the possible expression of all 13 aquaporin isoforms cloned in humans. Interestingly, we found that ARPCs expressed both AQP5 mRNA and mature protein. This novel finding prompted us to investigate the presence of AQP5 in situ in kidney. We report here the novel finding that AQP5 is expressed in human, rat and mouse kidney at the apical membrane of type-B intercalated cells. AQP5 is expressed in the renal cortex and completely absent from the medulla. Immunocytochemical analysis using segment- and cell type-specific markers unambiguously indicated that AQP5 is expressed throughout the collecting system at the apical membrane of type-B intercalated cells, where it co-localizes with pendrin. No basolateral AQPs were detected in type-B intercalated cells, suggesting that AQP5 is unlikely to be involved in the net trans-epithelial water reabsorption occurring in the distal tubule. An intriguing hypothesis is that AQP5 may serve an osmosensor for the composition of the fluid coming from the thick ascending limb. Future studies will unravel the physiological role of AQP5 in the kidney.

Ascl3 knockout and cell ablation models reveal complexity of salivary gland maintenance and regeneration

Expression of the transcription factor, Ascl3, marks a population of adult progenitor cells, which can give rise to both acinar and duct cell types in the murine salivary glands. Using a previously reported Ascl3(EGFP-Cre/+) knock-in strain, we demonstrate that Ascl3-expressing cells represent a molecularly distinct, and proliferating population of progenitor cells located in salivary gland ducts. To investigate both the role of the Ascl3 transcription factor, and the role of the cells in which it is expressed, we generated knockout and cell-specific ablation models. Ascl3 knockout mice develop smaller salivary glands than wild type littermates, but secrete saliva normally. They display a lower level of cell proliferation, consistent with their smaller size. In the absence of Ascl3, the cells maintain their progenitor function and continue to generate both acinar and duct cells. To directly test the role of the progenitor cells, themselves, in salivary gland development and regeneration, we used Cre-activated expression of diphtheria toxin (DTA) in the Ascl3-expressing (Ascl3+) cell population, resulting in specific cell ablation of Ascl3+ cells. In the absence of the Ascl3+ progenitor cells, the mice developed morphologically normal, albeit smaller, salivary glands able to secrete saliva. Furthermore, in a ductal ligation model of salivary gland injury, the glands of these mice were able to regenerate acinar cells. Our results indicate that Ascl3+ cells are active proliferating progenitors, but they are not the only precursors for salivary gland development or regeneration. We conclude that maintenance of tissue homeostasis in the salivary gland must involve more than one progenitor cell population.

Characteristics of neurokinin A-induced salivary fluid secretion in perfused rat submandibular gland

Tachykinins such as neurokinin A (NKA) and substance P have been demonstrated to induce salivary fluid secretion in vivo. However, characteristics of salivary fluid secretion induced by tachykinins in salivary glands have not been well elucidated. In this study, the effects of the tachykinin NKA on salivary fluid secretion were investigated in isolated, perfused rat submandibular gland. NKA provoked salivary fluid secretion, which consisted of transient and sustained phases, in a dose-dependent manner. In fura-2-loaded dispersed cells of the rat submandibular gland, the doses of NKA in which induced salivary fluid secretion caused an increase in intracellular Ca(2+) concentration. When Ca(2+) was removed from the perfusate to examine the effect of Ca(2+) mobilization on NKA-induced fluid secretion, only the transient salivary fluid secretion occurred. When the gland was perfused with the Ca(2+)-free perfusate containing the intracellular Ca(2+) chelator BAPTA-AM, NKA failed to induce salivary fluid secretion. NKA also induced an increase in oxygen consumption, but which was reduced by the removal of Ca(2+) from perfusate. Salivary fluid is secreted via transcellular and paracellular pathways in acinar cells of salivary glands. To examine the contribution of paracellular pathway to NKA-induced salivary fluid secretion, the glands were perfused with a perfusate containing Lucifer yellow (LY), a cellular impermeable substance, and then were stimulated with NKA, which provoked secretion of LY in the saliva. These results suggest that the NKA-induced salivary fluid secretion is Ca(2+)-dependent and that the paracellular pathway contributes to the secretion.

Downregulation of NCC and NKCC2 cotransporters by kidney-specific WNK1 revealed by gene disruption and transgenic mouse models

WNK1 (with-no-lysine[K]-1) is a protein kinase of which mutations cause a familial hypertension and hyperkalemia syndrome known as pseudohypoaldosteronism type 2 (PHA2). Kidney-specific (KS) WNK1 is an alternatively spliced form of WNK1 kinase missing most of the kinase domain. KS-WNK1 downregulates the Na(+)-Cl(-) cotransporter NCC by antagonizing the effect of full-length WNK1 when expressed in Xenopus oocytes. The physiological role of KS-WNK1 in the regulation of NCC and potentially other Na(+) transporters in vivo is unknown. Here, we report that mice overexpressing KS-WNK1 in the kidney exhibited renal Na(+) wasting, elevated plasma levels of angiotensin II and aldosterone yet lower blood pressure relative to wild-type littermates. Immunofluorescent staining revealed reduced surface expression of total and phosphorylated NCC and the Na(+)-K(+)-2Cl(-) cotransporter NKCC2 in the distal convoluted tubule and the thick ascending limb of Henle's loop, respectively. Conversely, mice with targeted deletion of exon 4A (the first exon for KS-WNK1) exhibited Na(+) retention, elevated blood pressure on a high-Na(+) diet and increased surface expression of total and phosphorylated NCC and NKCC2 in respective nephron segments. Thus, KS-WNK1 is a negative regulator of NCC and NKCC2 in vivo and plays an important role in the control of Na(+) homeostasis and blood pressure. These results have important implications to the pathogenesis of PHA2 with WNK1 mutations.

Conditional overexpression of TGF-beta1 disrupts mouse salivary gland development and function

Transforming growth factor-beta (TGF-beta) signaling is known to affect salivary gland physiology by influencing branching morphogenesis, regulating ECM deposition, and controlling immune homeostasis. To study the role of TGF-beta1 in the salivary gland, we created a transgenic mouse (beta1(glo)) that conditionally overexpresses active TGF-beta1 upon genomic recombination by Cre recombinase. beta1(glo) mice were bred with an MMTV (mouse mammary tumor virus)-Cre (MC) transgenic line that expresses the Cre recombinase predominantly in the secretory cells of both the mammary and salivary glands. Although most of the double positive (beta1(glo)/MC) pups die either in utero or just after birth, clear defects in salivary gland morphogenesis such as reduced branching and increased mesenchyme could be seen. Those beta1(glo)/MC mice that survived into adulthood, however, had hyposalivation due to salivary gland fibrosis and acinar atrophy. Increased TGF-beta signaling was observed in the salivary gland with elevated phosphorylation of Smad2 and concomitant increase in ECM deposition. In particular, aberrant TGF-beta1 overexpression caused salivary gland hypofunction in this mouse model because of the replacement of normal glandular parenchyma with interstitial fibrous tissue. These results further implicate TGF-beta in pathological cases of salivary gland inflammation and fibrosis that occur with chronic infections in the glands or with the autoimmune disease, Sjögren's syndrome, or with radiation therapy given to head-and-neck cancer patients.

Apoptosis and expression of AQP5 and TGF-beta in the irradiated rat submandibular gland

PURPOSE

To evaluate the effect of X-ray irradiation on apoptosis and change of expression of aquaporin 5 (AQP5) and transforming growth factor-β(TGF-β) in the rat submandibular gland (SMG).

MATERIALS AND METHODS

SMGs of 120 male Sprague-Dawley rats were irradiated with a single X-ray dose (3, 10, 20, or 30 Gy). At the early and late post-irradiation phase, apoptosis was measured by the terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) method, and expression of AQP5 and TGF-β was determined by immunohistochemical staining.

RESULTS

At the late post-irradiation phase, increased apoptosis was evident and marked decreases of expression of AQP5 expression by acinar cells and TGF-β expression by ductal cells were evident.

CONCLUSION

Apoptosis after X-ray irradiation develops relatively late in rat SMG. Irradiation reduces AQP5 and TGF-β expression in different SMG cell types.

Aquaporins in secretory glands and their role in Sjögren's syndrome

Salivary, lacrimal and pancreatic secretions are known to account for multiple physiological functions. These exocrine secretions are watery fluids containing electrolytes, and a mixture of proteins, and can be stimulated by a number of agonists. Since water movement is involved in exocrine secretion, aquaporins (AQPs) have been hypothesised to contribute to fluid production in exocrine glands. This chapter will focus on the expression, localisation and function of AQPs in salivary and lacrimal glands and pancreas. The role of multiple water and ion transporters and channels in exocrine fluid secretion will also be reviewed. Finally, this chapter will address the potential role of AQPs in Sjögren's syndrome.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.